Tunable optical filters are useful in situations requiring spectral analysis of an optical signal. They can also be used, however, as intra-cavity laser tuning elements or in tunable detectors, for example. One of the most common, modem applications for these devices is in wavelength division multiplexing (WDM) systems. WDM systems transmit multiple spectrally separated channels through a common optical fiber. This yields concomitant increases data throughput that can be obtained from a single optical fiber. There are additional advantages associated with common amplification across the channels in an optical link and as a platform for dynamic channel/wavelength routing.
Tunable filters that operate in these WDM systems must typically be high quality/high finesse devices. Currently proposed standards suggest channel spacings of 100 GigaHertz (GHz) to channel spacings as tight as 50 GHz in the ITU grid; some systems in development have spacing of 20 GHz and less. Tunable filter systems that operate in systems having such tight channel spacings must have correspondingly small passbands when operating as monitors, receivers, and routing devices.
Typically, the design of the tunable filters is based on a class of devices generally referred to as Fabry-Perot etalons. These devices have at least two highly reflective elements defining the Fabry-Perot cavity. The tunability functionality is provided by modulating the optical length of the cavity.
Since these tunable filters are typically incorporated into larger systems offering higher levels of functionality and because the Fabry-Perot cavity must be modulated over distances corresponding to the wavelength of light that it is filtering, typically around 1,000 to 2,000 nm in wavelength, micro optical electromechanical systems (MOEMS) technology is typically used to fabricate the tunable filters. The most common implementation pairs an electrostatically deflectable membrane with a fixed reflector. Thin film technology is typically used to render the membrane and fixed reflector reflective. High quality or high finesse systems can require dielectric mirrors having greater than seven layers.
A common metric for characterizing the quality of tunable filter systems is the side mode suppression ratio (SMSR). This is the ratio between the magnitude of the lowest order mode in the spectral plot of the filter""s characteristic and the magnitude of the next largest mode, which is typically, but not necessarily, the next higher order mode.
Fabry-Perot cavity designs exist that maximize SMSR. Typically, the easiest approach is to use a confocal Fabry-Perot cavity. In confocal cavities, all modes are degenerate, i.e., the modes all coexist at the same frequency, or wavelength.
MOEMS confocal cavities, however, are difficult to manufacture on commercial production scales. Spacing between the deflectable membrane and the curved mirror can be difficult to control. Moreover, curved membranes can be difficult to manufacture with the required curvature. Finally, it is difficult to maintain the confocal configuration while tuning.
A more typical configuration for MOEMS tunable filter Fabry-Perot cavities is termed a hemispherical cavity or curved-flat. In such cavities, one of the reflectors is near planar and the other reflector is curved.
When hemispheric tunable filters are used, for example, the optical train surrounding the filter must be designed with the objective to control SMSR.
One solution to controlling SMSR used in some conventional MOEMS filter systems is to integrate the tunable filter into the larger optical system by locating it between two fiber pigtails; one fiber pigtail emits the optical signal to be filtered and the other fiber pigtail collects filtered optical signal after its transmission through the tunable filter. The tunable filter is oriented to be orthogonal to the axis extending between the fiber endfaces.
As optical systems are developed that allow for higher levels of finctionality in a single package, the alignment of the tunable filter element in the optical system becomes less trivial. This is especially true in systems utilizing free-space-interconnects between the tunable filter and other optical components in the system.
Improper or imprecise alignment can excite higher order modes in the optical filter train. These higher order modes are undesirable because they can cause confusion as to how many WDM channels exist in, for example, the received signal. It can also cause undesirable inter-channel crosstalk.
One of the easiest solutions to controlling SMSR contemplates the use of spatial filters. Higher order spatial modes, other than the TEM00 have generally larger modal volumes. As a result, pinhole apertures in the optical train and/or the use of single mode fiber may be used to control the side mode suppression.
Such solutions, however, have undesirable side effects. Apertures only address some of the modes. For example, the TEM20 mode has a substantial amount of power propagating along the optical axis. The use of single mode fiber suffers from similar drawbacks. Further, spatial filters degrade dynamic range and the integration of fiber into the optical train requires additional alignment steps and is orthogonal to achieving higher levels of integration. As a result, the best solution to improving the side mode suppression ratio is the robust design and manufacture of the optical train, including the tunable filter and the surrounding optics.
The present invention is directed to a method and system for optical train alignment where the optical train includes a tunable filter. Specifically, in the preferred embodiment, the SMSR of the tunable filter train is monitored while it is being actively aligned. Thus, active alignment techniques can be used to maximize the train""s SMSR performance.
The preferred solution is to control the alignment of the tunable filter and the surrounding optical train to minimize the degree to which the higher order modes are excited in the filter train. This involves both controlling the alignment and mode size of the beam that is coupled into the tunable filter.
In general, according to one aspect, the invention features a process for tunable filter optical train alignment. This process comprises detecting a spectral response of the filter train and aligning an optical fiber that transmits an input optical signal to the filter train during operation. Further, the tunable filter is moved relative to the filter train, in one embodiment, in response to a spectral response of the filter train. In operation, this can be achieved by physically moving the filter or alternatively its surrounding optical train. As a result, the alignment and spectral response of the tunable filter train are optimized. In the preferred embodiment, the alignment and SMSR optimization occur simultaneously with respect to each other.
According to a preferred embodiment, the step of aligning the optical filter comprises injecting a diagnostic signal into the filter train and detecting a level of the diagnostic signal that is coupled into the optical fiber. The endface of the optical fiber is then moved in response to the level of the diagnostic signal. Specifically, in the preferred embodiment, the optical fiber is aligned so that coupling efficiency is maximized. Further, the fiber is preferably aligned with respect to a back-reflected signal from the tunable filter. Specifically, the diagnostic signal is injected through the optical fiber and then a level of back-reflected light from tunable filter is detected. The optical fiber is aligned to this back reflection. In this way, the alignment of the optical train is optimized using an active alignment technique.
According to further aspects of the preferred embodiment, a side mode suppression ratio of the spectral response of the filter optical train is detected. The tunable filter is then aligned relative to the filter train so that this side mode suppression ratio is maximized.
In the preferred implementation, the spectral response of the tunable filter train is detected in real-time by scanning the tunable filter over a single frequency or near single frequency diagnostic signal. The level of transmitted signal as a function of time corresponds generally to the tunable filter""s spectral filtering characteristic. This characteristic can be analyzed for side mode suppression ratio and used to derive a feedback signal in the active alignment and/or positioning of the tunable filter relative to the rest of the optical train.
In general, according to another aspect, the invention also features a calibration system for an optical train including a tunable filter. The calibration system comprises a filter alignment system for positioning the tunable filter relative to the filter train. A signal generator is further provided for injecting a diagnostic signal into the filter train. A detection system is used to detect the diagnostic signal after interaction with the tunable filter and generate information concerning a spectral response of the filter train. Finally, a control system is used that controls the filter alignment system in response to the detected spectral response.
In one implementation, a broadband diagnostic signal is used in conjunction with a spectrum analyzer to determine the filter""s spectral response. Spectrum analyzers, however, tend to be inherently high loss devices. Thus, for the dynamic range required in some applications, this approach would be relatively slow.
In the preferred embodiment, the signal generator is a generally single frequency laser have a bandwidth of less than 1 MHz, for example. A tunable filter controller is then used that tunes the tunable filter across the spectrum of the diagnostic signal.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.